There are various configurations of linear motors, including generally flat motors, U-channel and tubular shaped motors. Different types of linear motors also are available, including brush, AC brushless, stepper, and induction motors. Common to most linear motors are a moving assembly, usually called a forcer, which moves relative to a stationary platen according to magnetic fields generated by application of current through one or more associated windings. The windings can be on the forcer or at the platen depending on the type of motor. For example, in a permanent magnet linear motor, a series of armature windings are mounted within a stage that is movable relative a stationary base plate or platen. The platen typically includes an array of permanent magnets configured to interact with the coils in the stage when energized with an excitation current. Alternatively, the magnets can be located in the stage with the coils situated in the platen. A closed loop servo positioning system is employed to control current through the windings. For example, current is commutated through coils of the stage with a three phase sinusoidal or trapezoidal signal in a closed loop feedback system. When such a linear motor is used in a positioning system, the relationship between the location of the stage and locations of the coils is utilized to control its operation.
Linear motors are increasingly being employed in manufacturing equipment. In such equipment, nominal increases in the speed of operation translate into significant savings in the cost of production. However, the cost of such equipment often plays a decisive role in determining which type of system will be employed. Linear motors are used in various types of systems, such as for positioning and moving items, including machining and gantry type systems. The systems often require moving items at high acceleration levels. In order to achieve such high acceleration, the linear motor must exert large forces upon the items to be moved.
Most linear motors are manufactured to follow a straight path and to be of a predetermined fixed length. This establishes the length of the armature, and consequently the number of armature windings. Linear motors do not produce an equal amount of torque at any singular position. For example, a one meter motor can have varying torque at any particular position. Variations in the linear motor or system employing the linear motor cause significant errors or significant difficulties in servo positioning using servo systems to position accurately or to control velocity very smoothly. These disturbances or errors manifest themselves in the servo system, which uses the motor as a device for putting out a constant force independent of position. If the force provided is too low due to errors associated with the linear motion system, the system is not able to hold position or control velocity of the linear motor.
Cogging and ripple errors provide many of the problems associated with varying torque in the motor and/or system. Cogging is associated with the change in reluctance and magnetic field strength at a given position. Ripple is associated with the back EMF of the motor produced by interaction of coils and magnets. Ultimately, the input signals have known wave shapes and provide a uniform force at any position. However, cogging and ripple errors in addition to many other effects cause distortion of the input signal. Other effects are associated with imperfections in mechanical windings, mechanical alignment errors, magnetic fields and mismatches with input signals. All of these errors show up as harmonic distortions of the back EMF waveform.